Cordycepin, or 3'-deoxyadenosine, is a derivative of adenosine nucleoside, the difference between the two is that the former is less than 3 in the ribose. The substance was first obtained from fungal Cordyceps, but it has now been synthesized chemically. Because cordyceps is similar to adenine deoxyribonucleosides (lack of oxygen at the top 2), many enzymes can not distinguish between the two. Therefore, cordyceps can participate in some biochemical reactions, such as RNA transcription.